1. Field of the Invention
The present invention relates to a magnetic resonance imaging (abbreviated as "MRI" hereinafter) apparatus for obtaining tomograms of desired sections of an object to be examined by utilizing nuclear magnetic resonance (abbreviated as "NMR" hereinafter). In particular, it relates to an MRI apparatus which enables multiple-slice (multi-slice) MRI in a short period of time.
2. Related Art
An MRI apparatus detects density distribution of atomic nucleus spins (referred to as merely "spin(s)" hereinafter), relaxation time distribution and the like of a desired section of an object to be examined through NMR and forms images of the desired cross-section of the object from the acquired data. A typical imaging technique therefor is the spin echo imaging technique.
In spin echo imaging, a desired slice is excited by applying a radio frequency (RF) pulse and gradient magnetic fields simultaneously, and then a 180.degree. RF pulse (referred to as "180.degree. pulse" hereinafter) is applied to acquire an NMR signal as an echo signal after a period of time from the 180.degree. pulse equal to the duration between the first RF pulse and the 180.degree. pulse. This basic sequence unit from the excitation to the acquisition of the echo signal, which takes a given repetition time Tr, is repeated while changing the intensities of phase encoding gradient magnetic fields (phase encoding) for every repetition, for a given number of times, for example, 256 times. The acquired signals are transformed through two-dimentional Fourier transform to afford data necessary for forming one image. By suitably selecting the echo time TE and/or the repetition time Tr, a T1-weighted image or T2-weighted image, which is useful for detecting lesional tissues, can be obtained.
However, the spin echo imaging technique requires a long measurement time because it takes time equal to the repetition time Tr multiplied by the number of the phase encoding steps (for example, 256). In particular, in order to obtain a T2-weighted image, which requires a particularly long repetition time Tr, it takes several to several tens of minutes.
A technique proposed for overcoming this drawback of the spin echo technique is the fast spin echo imaging technique (for example, refer to "Fast Spin Echo Imaging Technique", INNERVISION 7 (5): pp15-20, 1992 etc.). In a pulse sequence of the fast spin echo imaging, an excitation pulse for selecting a slice is applied and then several 180.degree. pulses are sequentially applied to obtain echo signals of the same number as that of the 180.degree. pulses. These echo signals are differently phase encoded and used as data for the same image. Thus, the repetition times can be reduced to a number equal to the number of data lines necessary for the reconstruction of a single image divided by the number of the echo signals. For example, if a fast 16-echo train spin echo sequence is used to collect data including 256 phase encoding steps, the necessary data for a single image can be collected by 256/16=16 repetitions. An image provided by the fast spin echo technique can have almost the same image contrast as one obtained by the spin echo imaging technique.
However, in the conventional fast spin echo imaging described above, the number of excitations by the 180.degree. pulses increases as faster imaging is attempted, and this may cause several problems. First, a pyrogenic effect on living body tissues known as specific absorption ratio (SAR) may be manifested. This effect is produced by repeated excitation of a same tissue of a living body by RF magnetic field and the extent depends on the intensity of a static magnetic field, the intensity, period and interval of the applied RF pulses, and the like. In the fast spin echo imaging, 180.degree. pulses of high magnitude are repeatedly applied within a very short period of time and the effect becomes pronounced in multi-slice imaging. A second problem is the magnetization transfer contrast (MTC) effect on the examined region of tissues. The MTC effect is that of the energy having excited the magnetization in binding water transferring to magnetization in free water, and degree of the effect varies depending on the distribution of free water and binding water. Thus, signals may be lowered depending on the tissue. Therefore, while image contrast different from that obtained by the conventional spin echo imaging and hence valuable for diagnostic purposes maybe obtained, contrast of lesional tissues is often lowered, e.g., signals from fat tissue are intensified. This MTC effect is manifested, e.g., in multi-slice imaging, when a gradient magnetic field for slicing is applied and excitation is performed by a frequency deviating from the resonant frequency (center frequency) of protons (off-resonance). In the fast spin echo imaging, this effect becomes particularly strong because the number of the RF pulses is increased.
The object of the present invention is to provide an MRI apparatus which solves the problems of the fast spin echo imaging technique mentioned above and enables fast multi-slice imaging.